Stereographic imaging system using open loop magnetomechanically resonant polarizing filter actuator

ABSTRACT

An apparatus and method for stereoscopic 3D image projection and viewing using a single projection source, alternating polarization, and passive eyewear. This approach is applicable to frame sequential video created using existing 3D graphics APIs, as well as other video signal formats, and is compatible with existing digital light processor (DLP) technology for both front and rear projection systems. An alternating polarizer in the form of a magnetomechanically resonant polarizing filter actuator is used to modulate the projected image. A preferred embodiment enables an existing DLP projection system to be enhanced with 3D capability.

CROSS REFERENCES TO RELATED APPLICATIONS

U.S. patent application Ser. No. 11/314,379, filed Dec. 21, 2005,entitled “STEREOGRAPHIC PROJECTION APPARATUS WITH PASSIVE EYEWEARUTILIZING A CONTINUOUSLY VARIABLE POLARIZING ELEMENT”; Ser. No.11/468,369, filed concurrently herewith, entitled “CLOSED LOOP FEEDBACKCONTROL TO MAXIMIZE STEREO SEPARATION IN 3D IMAGING SYSTEMS”; and Ser.No. 11/468,370, filed concurrently herewith, entitled “DYNAMIC PROJECTORREFRESH RATE ADJUSTMENT VIA PWM CONTROL” are assigned to the sameassignee hereof and contain subject matter related, in certain respect,to the subject matter of the present application. The above-identifiedpatent applications are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention provides a three dimensional (“3D”) imageprojection apparatus that can be combined with a single light sourceprojection system, and enables a viewer to perceive a three dimensionalprojected image while using passive eyewear. A polarization scheme,compatible with both front and rear projection digital light processorbased projection systems, as well as frame sequenced projection systems,is described herein.

2. Description of the Prior Art

FIG. 1 shows a prior art 3D imaging system that illustrates severalfundamental requirements for 3D image projection. Two two-dimensional(“2D”) images of one scene are displayed, such as by projectionapparatuses 101, one of which is slightly different than the second interms of a line of sight perspective. These perspectives typicallydefine a left eye view and a right eye view. This normally requires dualimage recordation in order to provide the two perspectives, or views, asdescribed above, of the same scene. However, such perspectives could bedigitally processed, or manually generated. FIG. 1 illustrates twoprojectors 101 one of which projects a left eye perspective of a scenewhile the other simultaneously projects a right eye perspective of thesame scene. Although simultaneous projection requires two projectors, itis possible to implement a single projector in a 3D imaging system byrapidly alternating the left and right eye perspectives duringprojection.

Another theoretical requirement of conventional 3D imaging systems is toexpose one of the projected perspectives to substantially only one ofeither the left or right eye and to expose the other of the projectedperspectives to substantially only the other eye, such that eachprojected perspective is seen substantially exclusively only by one eye.The term “substantially” is meant to convey that the image exclusivitybe sufficient to induce a 3D perception in the viewer. Thus, with a dualsimultaneous projection system, one eye of a viewer will besubstantially blocked from seeing the image content from one of theprojectors and the other eye will be substantially blocked from seeingthe image content from the other projector.

This blocking, often referred to as extinction, can be accomplished intwo steps. First, each of the simultaneously projected images ispolarized at a different polarizing angle by projection throughseparately angled polarized transparent media 102. The viewer wearspassive polarized eyewear 103 whose lenses are also offset polarizedsuch that one of the lenses will block a first one of the polarizedprojected images and the other lens will block the second one of thepolarized projected images. The physics of polarization, particularlyoverlapping transparent polarized media, is sufficiently known by thoseskilled in the art and is not described in detail herein. Prior artmethods of providing two different projected perspectival images to aviewer include coloring the projected images using red and blue colorcoding combined with passive eyewear having a blue and a red lens thatis worn by the viewer.

There have been many attempts to generate 3D image systems. We areconcerned here with 3D imaging in systems which use polarizationencoding of the left and right eye views, which may be implemented usinga switched system. Modern front and rear projection color imagingsystems, such as DLP technology, employ multiple color filters tosequentially project elements of a full color image onto a screen. Thesecolor filters are typically implemented as segments on a color filterwheel, which spins at a rate synchronized with the input video stream.Typically, this approach uses the three basic video imaging colors (red,blue, and green) in combination with a high brightness white lightsource. In order to facilitate white balance of the image and correctfor certain kinds of image aberrations, a transparent filter segment isoften incorporated into the color filter wheel, allowing white light topass through.

In order to modify these imaging systems so that they support thetransmission of stereoscopic three-dimensional images, it is necessaryfor them to provide separate left and right eye views. The separate eyeviews can be provided by separate image streams that are combined into asingle stream of digital image data and by alternating the projection ofimage data between these image streams to provide the left and right eyeviews to the viewer and using an additional filtering apparatus, whichmay or may not be part of the same color filter wheel used in theprojector. In this case, it may become necessary to synchronize thephase, frequency, and possibly other attributes of the rotating colorfilter wheel with an external stereoscopic imaging element. Thissynchronization is not necessarily always achieved simply by accessingthe electronic signals used to control the color filter wheel.

Published patent application US 2005/0041163A1 describes the use of asegmented polarizer attached to the color filter wheel inside a digitallight processor (“DLP”) projector. It does not describe any requiredrelationship between the projector lens optics and an alternatingpolarizer with respect to polarization sensitivity. Thus, the projectionlenses and other optics may corrupt the polarization encoded imagesignal. Details of the synchronization required between the filter wheeland polarization wheel are not described, nor is there any reference tothe distinction between frame sequential and other types of video input.This prior art will not work for all types of video input such as lineinterleaved video streams. The above-identified patent application isincorporated herein by reference in its entirety.

U.S. Pat. No. 5,993,004 describes a stereoscopic display with a spatiallight modulator and polarization modulator, using polarizationpreserving optics and special control signals for the modulation. As ageneral statement, this approach does not use alternating polarizationas our invention does. The above-identified patent is incorporatedherein by reference in its entirety.

Published U.S. patent application 2005/0046700A1 describes two videoprocessing devices which process at least four separate sequences ofvideo images for projecting multiple image views on a screensimultaneously. At a high level, this approach does not use alternatingpolarization as our invention does. The above-identified patentapplication is incorporated herein by reference in its entirety.

Published U.S. Application 2003/0112507 describes two embodiments forDMD devices, both of which use different rows or columns of the DMDdevice driven sequentially to provide different eye views of the sameimage. This approach is not related to the use of alternatingpolarization as our invention is. The above-identified patentapplication is incorporated herein by reference in its entirety.

Published U.S. application 2003/0214631 describes a projector with abeam splitter to produce two light paths, each of which passes through afixed polarizer and are later recombined with a special optical system.This approach does not use alternating polarization as our inventiondoes. The above-identified patent application is incorporated herein byreference in its entirety.

U.S. Pat. No. 1,879,793 describes the original motion picture projectionsystem (similar to those later used in IMAX 3D applications) in whichthe rate of film passing through the projector is synchronized in somefashion with an external polarizing wheel or slides. This approachrequires special film processing techniques. The above-identified patentis incorporated herein by reference in its entirety.

In the personal computer (“PC”) industry, liquid crystal display (“LCD”)optical shutter glasses have become the standard for cathode ray tube(“CRT”) and projector viewing for color 3D imagery. However, thisrequires active eyewear (with a miniature liquid crystal monitor orshutter in each lens), as well as requiring a battery and connection tothe data source for synchronization purposes. These solutions also tendto be expensive, are only practical for a limited number of users at onetime, and tend to induce eye strain after prolonged use. These glassestypically use the Display Data Channel industry standard contained inevery modern video adaptor card interface. This data channel signals theglasses that the PC has swapped its eye view.

In general, the prior art requires the projector to use internal opticswhich are polarization insensitive, since the light polarization must bemaintained from the filter wheel through the rest of the projectionpath. This means that special optics must be used, and polarizationsensitive coatings must be avoided, thereby increasing both thecomplexity and implementation cost.

SUMMARY OF THE INVENTION

One embodiment of the present invention includes an enhancement to a 3Dimaging system and uses light polarization to encode images for the leftand right eye views. The polarization encoding is generated by amagneto-mechanical voice coil actuator. A television system includingthis invention comprises an input for receiving image data that includesa plurality of image streams and a light projector for projecting theimage streams. The magnetomechanical oscillating filter includes anoscillating arm with attached polarization filters for polarizing theprojected image streams at different polarizing angles.

A method of the present invention includes receiving image datacomprising a plurality of image streams and projecting the image datawhile alternating between the image streams. A magnetomechanicallyoscillating polarizer filters frames projected from the image streamsdepending on which of either a left or right eye view is beingprojected.

In order to get maximum separation between the left and right eyeimages, and thus the best depth of field for 3D images, the systemshould ideally hold the polarizer at a fixed orientation relative to thelight beam while each frame is projected. Ideally, one would use thesmallest piece of polarizing film possible and place it at the smallestimage beam size within the system, to achieve uniform polarization ofthe entire image.

In order to address these issues, we have developed a new design for a3D projection system, as shown in FIGS. 3A-C. We use a magnetic bearingsystem to move a pair of small pieces of polarized film through theprojector's internal focal point. Since the image is sufficiently smallat this point, we achieve uniform polarization of the entire image.Further, we need to move the film by only a small amount to switchbetween polarization states. A working prototype moves the polarizingfilters film up or down by approximately one inch through an angle ofabout 30 degrees.

A multiplexed sensor array, illustrated in FIGS. 8A-C, can beimplemented for optimal extinction of the polarizer device. Becausethese are analog sensors, it can be incorporated into a linear feedbackcontrol system to achieve optimal performance and power efficiency. Mostpolarizing material available today has a nicely flat passband in thewavelength range of 400 nm-700 nm with approximately 60-70%transparency. One embodiment of the present system uses visible lightsources to achieve linear feedback signals. We use an LED device with anoutput wavelength of 660 nm because the output spectrum of theUHP/SHP/P-VIP/Xenon arc lamp typically used in projectors is verydeficient in the red range (roughly 600 nm and above). In this way,along with adequate mechanical shielding, interference from the imagesfrom the projector itself will not be able to cause signal interference.

It is an object of the invention to provide a method and apparatus forgenerating stereoscopic three-dimensional images on front or rearprojection imaging systems, such as digital light processor (DLP) orsimilar apparatus. These solutions require a signal from the videosource to synchronize the imaging system projector. Typically, thissignal is derived from the standard video signal interface on a personalcomputer (“PC”). The present invention does not require that the 3Drecordation be done with any specific equipment or number of cameras,only that two perspectives be obtainable or derivable from image dataand are capable of being projected for display.

It is an object of the present invention to provide three dimensionalimaging at low implementation cost, compatible with many different kindsof video input or image data sources. Further, this invention providesthe means for multiple viewers to perceive the image at the same time,from a range of viewing angles. The present invention is compatible withlegacy projection devices.

A method of the present invention includes receiving image data thatcomprises a number of image streams, typically transmitted digitally andsequentially and each containing images having defined boundaries. Theseare usually referred to as data frames. A digital projection apparatusand system using a light source is capable of receiving this image dataand projecting the represented images toward a screen for viewing by anynumber of observers who sit on either side of the screen depending onwhether the system is a front or rear projection system. The image canbe still, motion picture, or computer generated, color or black andwhite. For generating a 3D effect, modern projection systems typicallyalternate projection of images that contain left and right eye views.This usually, though not necessarily, means that individual frames arealternately transmitted which contain alternate left and right eyeviews. Switching between left and right eye view projection need notalternate with each frame, though the left-right images should switch ata rate fast enough to not be detectable by a viewer. Thus, the imagestream may swap frames faster than swapping left/right eye views.

The individual left and right eye projected images are polarized atdifferent, complementary polarization angles before the projected imagesreach the screen using a unique magnetomechanically actuated alternatingfilter that is synchronized with the transmitted image data. Passivepolarizing eyeglasses are worn by a viewer, wherein the left and righteye lenses are polarized relative to the alternating filter polarizationsuch that selected ones of the polarized projected images will beblocked from view due to polarization extinguishing. This induces anexperience in the viewer of watching three dimensional still images, 3Dmotion picture images, or computer generated images, still or in motion.

An apparatus for performing the method described above includes aprojection television system or other imaging apparatus thatsequentially projects images using a light source. An input receives theimage data which includes synchronization signals to indicate whether aleft or right eye view data is being received. This received image datais projected using a light source projector. A voice coil actuator motordriven alternating filter positioned in the path of the projectedstereographic image data separately polarizes the projectedstereographic image data according to whether left or right eye data isbeing received and is synchronized by detecting the synchronizationsignals.

These, and other, aspects and objects of the present invention will bebetter appreciated and understood when considered in conjunction withthe following description and the accompanying drawings. It should beunderstood, however, that the following description, while indicatingpreferred embodiments of the present invention and numerous specificdetails thereof, is given by way of illustration and not of limitation.Many changes and modifications may be made within the scope of thepresent invention without departing from the spirit thereof, and theinvention includes all such modifications. The figures below are notdrawn to any accurate scale with respect to size, shape, angularrelationship, spatial relationship, or relative positions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art dual projection system for 3D imaging.

FIG. 2 illustrates a basic prior art DLP projection system.

FIG. 3A illustrates an embodiment of the present invention implementedwith the example DLP system of FIG. 2.

FIG. 3B illustrates an embodiment of a magnetomechanical oscillator.

FIG. 3C illustrates an embodiment of the present invention.

FIG. 3D illustrates a stereo signal synchronized with left and right eyeimage data.

FIGS. 4A-B illustrate physical considerations for sizing the polarizingfilter mechanism.

FIGS. 5A-D illustrate circuitry for the open loop and closed loopembodiments and electrical waveforms for an embodiment of the presentinvention.

FIGS. 6A-C illustrate the polarized filter and its movement.

FIG. 7 illustrates the sensor configuration.

FIGS. 8A-C illustrate an embodiment of the present invention.

FIG. 9 illustrates an embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention provides 3D viewing using passive eyewear, thusreducing cost and avoiding eye strain and color perception issuesassociated with various alternatives. It can be used with a singleprojection source based on popular DLP technology (or relatedtechnologies including GLV), and may be realized as either an integralpart of a projection system or as an add-on peripheral or stand that canbe placed in front of a projector. This invention takes advantage of theprior art 3D interface support provided in popular software packages,such as the OpenGL™ or Direct3D™ application programming interface(“API”), which includes variants such as java3D™. This interface iscompatible with the vast majority of 3D image software and programs inuse today. All of these APIs generate digital content with left/righteye perspectives, for use with alternative technologies such as theactive shutter glasses described previously. This content can be usedwithout modifications by our proposed invention. Synchronization isprovided by the device driver and left/right content is providedautomatically in these APIs. A user need only select OpenGL™ orDirect3D™ to render in stereo and it places the image streams in memoryconcurrently. The device driver then transmits the image data to atarget device (two projectors, one projector with page flipping, oneprojector with line interleaved stereo, for example). Other types ofdigital content may need to be preprocessed to generate left/right eyeviews compatible with 3D imaging techniques. For some types of contentsuch as digital movies or electronic images this processing is fairlystraightforward (other types of content, such as broadcast television,require additional, more complex processing to enable 3D viewing).

Referring to FIG. 2, illustrated are the basic principles of an exampleDLP system 209. A white light source 201 is focused through a condensinglens 202 which then passes through a rotating color filter wheel 203.The filter wheel may contain the three principle colors used to createvideo and graphics images (red, green, and blue), or different colors.The present invention is not limited to using only a rotating colorfilter wheel. Other technologies may exist or may be developed whichprovide a directed light beam containing a projected image (such asdigital frame sequential images, stereographic or not) that can beimplemented with the present invention. The proposed invention isgenerally applicable to any light source projection system, typically asingle light source that does not rely on polarization to create theimage itself. Restated another way, the polarization of the light outputof the imaging device used should be randomized. The presently proposedinvention uses polarization techniques to produce 3D effects, therefore,projections systems that rely on polarization techniques themselvesmight interfere with polarization implementations in the presentinvention.

As shown in FIG. 2, the beam passes through the color filter wheel 203,through a focusing lens 204, and illuminates a DMD 205 which is drivenby firmware from a video source, such as from a personal computer, DVD,a stored format, or a television signal, among others. Other possiblevideo sources include grating light valve and phase change displaytechnology.

The image content processed by the DMD is synchronized with the rotatingcolor filter wheel such that the red content of the desired imageilluminates the DMD when the red filter segment is aligned with the DMDwhile the focused light passes through it. The green image contentilluminates the DMD when the green filter is aligned with the DMD, andso on. Thus, for each image frame that is projected by such a DLPsystem, the color filter wheel and DMD operate together to sequentiallyproject several color planes for each image frame. The sequential partsof an image are then focused through additional projection optics 206onto a screen 207 to create a suitable 2D image, which may be a stillimage or motion picture. Images projected by a front projection systemwould be viewed from the same side 212 of the screen 207 as theprojection equipment. A rear projected image would be viewed from theside 211 opposite the projector equipment.

If the three color content is processed by the system shown in FIG. 2 ata sufficiently high frame rate, measured in frames per second, it willcause an observer to perceive a full color stable 2D image on thescreen. There are many variations of this technology, including systemswhich use 2 or 3 DMD devices to create the final image, and systemswhich employ both rear projection and front projection techniques.Details of the optical elements used in the beam path, such as thecondensing lenses and projection lenses, may also vary, and are notessential parts of the present invention. The dual projector apparatusshown in FIG. 1, for example, could be composed of two DLP projectors.Due to its performance and low cost, DLP is becoming a preferredtechnology for many large screen projection televisions, portablepersonal computer projectors, and similar applications.

FIG. 3A illustrates a preferred embodiment of the present inventionincluding a magnetomechanically oscillating polarizing filter 304 whichis mounted in front of an existing DLP projector 303, which may beimplemented as a front or rear projection system depending on which sideof the viewable screen 306 a viewer sits. Variants of the presentinvention allow for use with rear projection systems and for integratingthis invention internally to the projector. In a time divisionmultiplexed (“TDM”) video stream, for example, a PC transmits pageflipped stereo images, compatible with industry conventions such as theAPIs discussed previously. Page flipping refers to the sequentiallytransmitted alternating left and right eye views, or frames, whosetransmission speeds, or refresh rate, may vary from several to hundredsof Hertz. The left and right eye views are also often separatelyreferred to herein as image streams, as parts of the transmitted imagedata, even though they operate together to generate a 3D effect.

Note that there may not be any additional optics in the light pathbeyond the polarizer unless they preserve the polarization state of thelight. This is an important consideration if the magnetomechanicallyactuated polarizer is to be integrated within the projector. Theresulting image must also be projected on a screen 306 or other surfacewhich preserves polarization of the reflected light, or retracted lightas in rear projection systems. Such screens are commercially available,for example by ScreenTech™, of Hamburg, Germany, and Da-lite™ of Warsaw,Ind., USA. We also note that our invention may apply to other types ofimage projection technology besides DMD/DLP; for example, the recentlyproposed grating light valve (“GLV”) technology. GLV is an alternativeto DLP and other light engine projection technologies, in which acombination of diffraction grating and liquid crystal technology is usedto generate 2D images suitable for either rear or front projectionsystems.

With the present invention, passive polarizing eyewear can be used toview the full color 3D image(s). The proposed invention is intended tobe used with the industry standard linearly polarized 3D glasses havingan orthogonal polarization orientation (with 0° centered between leftand right eye): left eye −45° with respect to 0°, and right eye +45°with respect to 0°. Obviously, these angle orientations are not arequirement of the present invention but are selected merely forindustry compatibility. The present invention can be easily adjusted forimplementation using other angular relationships. It will be apparent tothose skilled in the art that minor adjustments to the invention willallow the use of passive eyewear with lenses having differentpolarization orientations as well as circular or elliptical polarizationstates. Circular and elliptical polarized passive eyewear iscommercially available.

It is well known that given a sufficiently fast video refresh rate, thehuman eye's persistence of vision will cause it to perceive a true color3D image, given an appropriately presented series of 2D images. In thisway, an existing DLP projector can be upgraded to project 3D images, byimplementing an external device to be placed at the output aperture ofthe projector, thereby requiring no modification to the originalprojector hardware or firmware. Note that frame sequential video signalsmust be used in order to achieve this effect with the present invention.Frame sequential video signals describe a time division multiplexedsignal of alternating left-eye designated and right-eye designatedimages. This means that individually transmitted images eachrepresenting a left or right eye view are sequentially received andhandled by the projection system. Although it may be ideal to alternatea left and a right eye view with each projected frame, as is the intentof the design of many digital stereographic systems, other designs mayalso produce a substantial 3D effect by taking advantage of the humaneye's persistence of vision. For example, some other sequential numberof frames may be transmitted for a right or left eye view prior toalternating frames for the opposite eye view.

Video Input Signal

The incoming stereo signal (e.g. 301 of FIG. 3A) typically is one thatis purposely produced for 3D stereo imaging, e.g. computer softwarevideo shot with dual lens cameras. However, the incoming image data cancome from a source such as a video game, PC, or digital television data.A stereo VGA signal and added information, such as DDC, HDMI, High Def,Multi Media Interface, and Y—Pr—Pb from digital cable boxes and DVDplayers, are also suitable. A minimum requirement for the presentinvention is an input containing stereo video data, which means thatdual left/right images can be obtained, derived, or processed from thevideo data. Video data can also be transmitted via packets, frames, orcells wherein header information can be used to indicate left or righteye content in the payload. In such an implementation, a 3D movie can betransmitted over the internet, and stored indefinitely, or projected forviewing as it is received, such as in real time video streaming. Someindustry standard signals, such as HDMI, would need to implement apreliminary circuit for extracting the sync signal (page flip signal)from the incoming video data. Thus, an HDMI input stream would work withthe present invention using a page flipping extractor whose output isprovided to the polarizer apparatus. HDMI input is typically providedwith page flip information on the input data. For the presentlydescribed embodiments, we assume that the incoming video signal is anindustry standard stereo VGA signal, those these standards may evolveand change over time. The scope of the present invention contemplatesimprovements in video imaging technology and its data content.

Embedded in the VGA standard is a “DDC” capability which is a lowbandwidth digital message interface implemented typically with abidirectional serial bus, to send page flip signals in parallel with theleft/right images indicating which of either a left or right image iscurrently being transmitted.

Industry standard protocols such as stereo VGA provide fixed known ratesfor the incoming frames, e.g. 60 Hz, 85 Hz, 100 Hz, or 120 Hz, where thestereo rate is half of that, thereby transmitting half as many of eachof the left and right eye frames/images per second. Many computerprograms also provide digital stereo image sources such as video games,architectural graphics programs, CAD programs, and medical imagingprograms, as examples, which contain stereo VGA signals.

With respect to FIG. 3A, an incoming stereo video signal (e.g. TDM framesequential) 301 is received by the conventional DLP projector 303 whichthen decodes and projects the video image. In parallel with the DLPprojector, the present inventive method and system receives the incomingstereo video signal and extracts, via field converter 302, the stereosynchronization signal 307, i.e. the page flip signal, embedded thereinand provides it to the magnetomechanical polarization filter 304. Thestereo synchronization signal indicates which of the two stereo imagesis present in the video stream at any instant of time. In the presentinvention, each of the two stereo image streams will be modulated as aleft eye designated or right eye designated. The extraction circuit 302is a well known VGA field converter circuit implementing the well knowDDC page flip protocol. eDimensional™, Inc. of West Palm Beach, Fla.,provides circuits for connecting to a stereo VGA signal which thenoutputs the page flip signal together with the video stream. The wellknown DDC algorithm can also be manually implemented by executing it onany of a variety of processors.

FIG. 3B illustrates, at a high level, the magnetomechanically actuatedpolarization apparatus 304. A motor 320 oscillates an attached rigidlever arm 323, between positions 324 and 325, which has attached theretoleft 322 and right 321 polarized filters.

With regard to FIG. 3C, showing in greater detail the apparatus 304, thepage flip signal arrives at input 353 activating an AC control andamplifier drive circuit 342 having two complimentary outputs 355,forming an AC current source which is coupled to and actuates a magneticcoil 352 within a larger fixed magnetic field generated by a pair ofparallel permanent magnetic plates 343 and 344. The alternating currentin coil 352 causes an alternating magnetic orientation in the coilcausing it to move a lever arm 351 up and down on each current cycle. Weattach the rigid lever arm to two small pieces of polarizing film 347and 348, and to pivot 354, with appropriate polarizing orientation onthe top and bottom film sections. In this way, the film is moved throughthe projector focal plane 349 (not shown precisely to scale or position)to encode left and right eye images with predetermined angles ofpolarization. Because the coil 352 is disposed in a static magneticfield generated by permanent magnets 343 and 344, the alternatingcurrent in the coil will drive its movement up and down, thereby causingthe polarizing filter arm 351, with attached pair of polarizing filters347 and 348 to similarly oscillate up and down around the pivot 354. Thepermanent magnet magnetic bearings 345 and 346 are prefabricated to havea magnetic flux strength resonant at the desired operating frequency,for example 60 cycles/second for a standard video stream. Thisprefabrication is a result of trial and error measurement of the energyconsumption of the actuator system using ammeters while adjustingmagnetic field strength of solenoids which are used in place of themagnets 345 and 346, as further explained below. The effects of gravityneed to be taken into account in selecting and/or tuning the fieldstrength of these magnets as the downward motion of the polarizer armwill be positively accelerated due to gravity and the upward motion willbe negatively accelerated to due to gravity.

These cushioning magnets 345 and 346 are optionally replaced, in analternative embodiment, with air core solenoids (illustrated in FIG. 9)that provide scalable duty cycle magnetic flux strength tuning. Byadjusting the current through the air core solenoid cushioning magnetswhile monitoring the coil current, an ideal cushion magnet flux can bedetermined because the motor coil current will reach a minimum amperage.Comparable magnetic flux strength permanent magnets then can be used toreplace the solenoids. Mechanical stops are provided at the top andbottom of the level arm's travel distance (not shown). As the coildriven oscillation of the filter arm 351 approaches stationary magnets345 and 346, attached magnet 350 dampens the movement of the filter armby decelerating its movement, preventing actual contact between themagnets, due to the increasing repulsive force exerted by the N-N poleand S-S pole proximity. The bottom permanent magnet 345 can befabricated with greater magnetic field strength so that the effects ofgravity can be compensated and the oscillating system is more symmetricwith respect to magnetic field effects. This results in a truemagnetomechanically resonant system. Controlled annealing of the magnetscan be used to selectively adjust their field strength, as measured inoersteds. The procedures for controlled annealing of magnets is beyondthe scope of the present invention and is not further discussed. Theinteraction of these fields also serve to steady the arm 351 when it isresting in an “off” state.

Achieving a 60 Hz magnetomechanical resonance results in a system thatrequires very little energy to drive. Without such resonance, it mayrequire in the range of approximately 50-100 watts to maintain 60 Hzresponse, while a magnetomechanically resonant system requiresapproximately 4 watts to drive at a steady state. This resonance can beachieved by replacing the permanent magnet cushions with temporary aircore solenoids coupled to a variable current source. The known specs ofthe solenoids, such as dimensions and number of windings, etc., current,can be used to determine the strength of generated magnetic flux. Vendorprovided permanent magnets having a magnetic flux equivalent to thetemporary solenoids can then be obtained and used in place of thesolenoids.

There are a number of additional features associated with thisinvention. For example, we note that if the projector is orientedhorizontally, then the lever arm will have a negative bias due togravity at the lower half of its travel distance. We have measured thearm's impulse response to confirm this and have compensated for this byadjusting the strength of the lower magnetic bearing 345 (this is alsoaccomplished by controlled annealing of the magnet as is mentionedabove). This could also be addressed by mounting the bearings sideways,though this may not be compatible with space constraints in existingprojectors. Also, we note that since the polarizer is moving at anangle, there will be some crosstalk when a linear polarizer, 347 or 348,is not centered on the image 349. This can be addressed by usingcircular polarizers, which are not sensitive to orientation but aresomewhat more expensive. Alternatively, we have been able to use linearpolarizer segments cut at a small bias angle of approximately threedegrees to reduce ghosting in the image. Thus far, we have mostlydescribed an open loop system with a moving coil that allows us to movethe polarizer filter with low energy expended.

Field Converter

With reference to FIG. 3A, the input signal is received by a fieldconverter 302 that extracts the page flip signal, or a videosynchronization (“vsync”) signal, as described above, which indicateswhich of left or right eye data, or image stream, is present in theimage data. The video data continues to be provided to the DLP projector303 while the page flip data 307 is used by the circuitry 304 of thepresent invention. The output signal (stereo signal) 307 of the fieldconverter is illustrated in FIG. 3D 360 and alternates between anindication for left eye view “L” and an indication for a right eye view“R” 361, which is simply a high, logical 1, and low, logical 0, voltagesignal. The square wave 360 indicates, for example, that the logical “1”(or higher voltage level) corresponds to right eye data in the videostream. A logical “0” (lower voltage level) indicates a correspondingleft eye image being transmitted in the image data. In an idealembodiment, the left and right eye image data alternates with eachindividual frame, but is not required in to produce an effective 3Dappearance.

A preferred embodiment of the present invention implements positive edgetriggering, or edge sensitive triggering, to detect the requiredsignals. An alternate embodiment could use level sensitive triggering,in which the signal is switched based on its amplitude crossing apreselected threshold. Level sensitivity implies variability induration, since the signal amplitude levels can drift or move because ofeffects such as noise and ground shifts. Thus, there must be a definedtolerance around the specified shift levels to account for these factorsin a practical design.

FIGS. 4A-C, illustrate characteristics of the servo device design, i.e.the motor element 320 in FIG. 3B. With respect to FIG. 4A, a set ofapplicable equations for the servo device system will help to determinethe amount of torque 407 that the motor will generate. In this figure,the motor is represented by a pair of permanent magnet plates 401 and402 that provide a static magnetic field between them. A coil 403 issuspended in the static magnetic field between the magnet plates and iscoupled to a pivot 404. As is well known in the art, the magnitude oftorque is equal to 2F/D sin(θ)=IBA sin(θ), in which D is the length ofthe lever arm, F is the linear force, and sin(θ)=1 in this instancebecause the coil is oriented orthogonally in the magnetic field; A isthe number of turns·radius (avg area of the coil); I is the currentthrough the coil wire; and B is the flux linkage of the magnet poles Nand S.

With respect to FIG. 4B, illustrated is an ideal balanced polarizer armcompensated for gravity using the motor torque drive provided by coil440 in a static magnetic field, as described above, and cushioning ofthe polarizer arm provided by cushioning magnets 442 and 443. In theillustration shown, there is an upward force F₁ 447 on the polarizer armand a downward force F₂ 448 aided by gravity, ½ AT², (A=distance,T=time) which must be added or subtracted from the applied torque force,F·radius, wherein F (force) is equal to (the polarizer mass)·gravity.The polarizer mass is the mass of the polarizer films 445 and 446. Thepolarizer arm, attached to sections of polarizing film 445 and 446, isfixed by a pivot 441. The cushioning magnets used 442 and 443 are N35Iron Boron (NdFeB) with residual flux of 12,000 gauss. These cushion thearm magnet 444, which is typically an off the shelf magnet whose fieldstrength is not adjusted via annealing or other techniques. In theequations illustrated in FIG. 4B, A=area of effective magnet faces;L=thickness of magnets; X=distance between magnets; R=radius of magnetsassuming they are discs. μ₀ is the relative permeability of air. B₀=fluxat magnet face in teslas; B_(r) (residual flux)=12,000 gauss;B_(h)max=35 mega gauss oersteds (a merit value of magnetic materialrelated to field strength).

The resulting dimensions of a working prototype, which can be scaled tosizes appropriate for various projector profiles, are as follows: eachof the polarizer films are approximately one inch square; the polarizerarm, from pivot to outside edge of polarizing film is approximately 2¼inches; the cushioning magnet dimensions are approximately ¼ in. dia.1/16 in. thick discs; the coil is approximately a ¾×1 in. torus; and thefield magnets are ¼w×2 in.

Open and Closed Loop Embodiments

FIG. 5A illustrates a MOSFET power inverter circuitry (element 342 inFIG. 3C) that drives the servo motor device in one embodiment of thepresent invention. The devices used in this circuit are n channel powerMOSFETs appropriately sized for the motor device. The fundamentalfrequency of the open loop circuit depends upon the videosynchronization signal (page flip) rate. When this page flip signal isat a high level (“1”) the power MOSFETs 545 and 547 are gated on andcurrent flows through the coil 553 in a direction from the upper leftMOSFET 545 to the lower right MOSFET 547. A logic low (“0”) page flipsignal then gates off the power MOSFETs 545 and 547 and activates theother two MOSFETS 546 and 548. Because the transformer windings arecoupled out of phase, current flows from the upper right MOSFET 548 downthrough the coil, in an opposite direction, to MOSFET 546. Thisalternating current flow through the coil drives the oscillation of thepolarizer arm as explained above. As illustrated in FIG. 5D, the pageflip signal (an excitation voltage for this circuit) is a square wave590, the coil current increase and decrease is linear 591, and themotion of the polarizer arm is sinusoidal 592.

FIG. 6A illustrates the approximate polarization angles of the filters602 and 603 as they are mounted on the lever arm 601 and the LED opticalsensors 604, 605 for detecting the lever arm in position forpolarization filtering of the image, shown in FIGS. 6B and 6C. Theaddition of the optical sensors adds a feedback element to the open loopembodiment described herein.

The filters are polarized at an angle such that when each filter is infinal position in the image beam 607, the effective relativepolarization of the image is orthogonal as between the left and rightfilters, as illustrated in FIGS. 6B and 6C. Obviously, thesepolarization angles can be reversed so that, for example, the leftpolarizing filter 602 is in a horizontal relationship with the projectedimage. When the lever arm 601 is fully in the “up” position, theprojected image 607, which is projected from the image stream thatcontains right eye frames, passes through the right filter 603 as shownin FIG. 6B. When the lever arm is fully in the “down” position, theprojected image 607 which is projected from the image stream thatcontains left eye frames, passes through the left filter 602 as shown inFIG. 6C. This illustrates the oscillating motion of the filters andselective projection and filtering of frames containing left and righteye image data.

Referring again to FIGS. 5A-C, to implement one embodiment of thepresent invention wherein sensors are used detect a position of thepolarizing filters, the drive current to the coil 553 is shut off whenthe polarizing filter achieves one of its filtering positions, therebyblocking its corresponding LED and triggering the corresponding sensor,and remains off until the next transition of the page flip signal inputwhich is transmitted to two inputs 541. There are two stages ofcommutation, as described below. With reference to the description ofthe circuitry shown in FIGS. 5A-C and in FIGS. 8A-C, it is well known tothose skilled in the art that electronic signal activation andtransmission can be of either positive or negative polarity depending onarbitrary circuit design and selection of devices, such as npn or pnptype transistors. Hence, the polarities of the illustrated waveforms andtheir descriptions herein are not critical to the correct operation ofthe present invention. Thus, the polarities of signal waveforms andcorresponding description may be reversed as between separate figuresherein. It is understood by those skilled in the art that a signal whichindicates the occurrence of an event may arbitrarily be selected as apositive or negative going signal and can be appropriately designed andinterpreted in any particular implementation.

Referring to FIG. 5C, during interval 1 583 switches 545 and 547 are on,and the actuator is accelerating in a first direction until a sensor isinterrupted. The interrupted sensor transmits at 560 a signal 580 to theone shot timer, illustrated in FIG. 5B as LSI 21, at input pin 5. Theone shot timer output, illustrated in FIG. 5C, is activated 582 for afixed amount of time, illustrated as interval 2, disabling the gatedrive signal to the MOSFETs by outputting at pin 1 564 the fixed timepulse which is transmitted to four inputs N at devices 542, 543, 550,and 551. These devices are MOSFET gate drive integrated circuits havinglogic level input and high current output with inverting andnon-inverting varieties. Example part Nos. include Texas InstrumentsUCC27321 and UCC 27322 Integrated Circuits, which are used in thisembodiment. This off time of the gate drive signal is illustrated asinterval 2 584. At this point the actuator is traveling due to itsmomentum. With regard to interval 3 585, the page flip signal changespolarity causing switches 546 and 548 to turn on, while 545 and 547 turnoff. While idealized drive transformers are shown, it should beappreciated by one skilled in the art that a number of gate drivetopologies can be employed, such as floating power supplies with opticalisolation for control, charge pumps, or custom H-bridge drive devicessuch as the IR2127S, from International Rectifier of El Segundo, Calif.

With reference to the waveforms illustrated in FIG. 5C, interval 1 583is the time window when the arm is traveling. Interval 2 (Tset) 584 isthe fixed time window when the drive current is off—the fixed off time.Interval 3 585 is the “travel time” when the polarizer arm is in motionmoving toward its opposite position until it triggers a sensor. Thenthere is another fixed off time, interval 4 586, while waiting for thedisable signal to turn off.

In the closed loop embodiment, a sensor will detect a polarizing filtercausing it to feed a pulse 560 to the LSI (LS 121, a one shot timer)chip. There will be two pulses provided per cycle, one during pull andone during push. They can be combined via a logical OR circuit toprovide a signal to the LSI. The disable signal corresponds to theoutput 564 of the LSI. The output pulse 564 is a fixed width pulseproportional to the RC circuit comprising a capacitor 561 and a resistor562 attached to it as shown, using a 250 KΩ resistor and a 1 μFcapacitor with a 5V power supply 563. These are devices preselectedappropriately for the overall system characteristics. The output of theLSI 564 is fed to the four enable signal inputs (N) on the driver chips542, 543, 550, and 551. When the disable signal is high the driver chipsare driving current to the MOSFETs through pulse transformers 549 and544, when the LSI output 564 goes low it pulls both drivers to the offstate. The output of the LSI is a fixed off time, when the sensor isinterrupting (filter is in proper position) it disables the power to thecoil via the one shot timer ship and its output to the driver chips. Ifthe page flip happens to change, nothing will happen until the constantoff time expires. This MOSFET circuit can be implemented within element342 of FIG. 3C. The page flip signal drives this mechanism at systemresonant frequency with minimal use of power.

Referring to FIG. 7, a functional side view illustration shows anexample phototransistor and photosensor set up for implementing each ofthe two sensor portions of the present embodiment. The left and rightpolarizer filter 701 travels between one of two pairs of an LED 703 anda photosensor 704. A fixed polarizer film 702 is located between thephototransistor and the LED of each sensor and has a polarization angleorthogonal to the polarization angle of either the corresponding left orthe right polarization filter on the oscillating polarizer arm when thearm achieves its corresponding terminus point in its oscillating travelpath. When the oscillating polarized filter passes between the LED andthe photosensor, the LED light will be blocked due to the orthogonalcooperation between the oscillating polarizer filter and the fixedpolarizer filter. This blockage of the LED 703 emitted light is detectedby the photosensor 704 and generates a signal, as described above whichcan be arbitrarily designed as either a positive or negative goingsignal, indicating that the oscillating polarizer is in position.

FIG. 8A illustrates an overall view of an embodiment of the presentinvention. Motor apparatus 803 drives rigid arm 804 in oscillationbetween sensor 1 807 and sensor 2 806. The rigid arm has attachedpolarizing filters 808 and 809, and a permanent magnet 805 forcushioning interaction with solenoids 801 and 802.

As shown in FIG. 8A, sensor 1 807, because it is blocked by thepolarizing filter, is, for example, outputting a low voltage signal,approximately zero, and sensor 2 806 is outputting a high signal. Itwill be obvious to those skilled in the art that the signaling can beselectively designed to output a high, logical 1, signal when the sensoris blocked and a low signal when it is not blocked.

With reference to FIG. 8C, depending on the state of the PF signal, thesensor 1 or sensor 2 output will be multiplexed out, via multiplexer840, and used as the feedback signal 844, waveform shown at 823, whichis coupled to input 852 of the motor amplifier 848 together with thepage flip signal 849 forming a summing junction at the input. Thisfeedback signal 823 is shown in FIG. 8B.

With regard to the linear differential output motor amplifier 848, itsoutput A+B, 850 is proportional to the difference in the input voltageat the + pin 852 and the fixed reference voltage 845 at the − pin 853.The output of the motor amp is coupled both to the air core solenoids801 and 802 and to the voice coil actuator 803 (motor) in one embodimentof the present invention. In another embodiment, the PWM embodiment, themotor amp is coupled only to the voice coil actuator. These signals tothe solenoids create a force against the polarizer arm permanent magnets805 (driven by the angular voice coil actuator) to provide a dampingeffect for the arm.

Referring to FIG. 8C, the page flip signal is fed into the mux control843 and the motor control amplifier 849. The page flip signal,illustrated at 824, operates to trigger multiplexing, via multiplexer840, the appropriate upper or lower sensor output, provided to themultiplexer at inputs 841 and 842, depending on the state of the pageflip signal (hi or low). The sensor outputs are illustrated at 821 and822, with the multiplexer output shown at 823. The feedback signal 844having a waveform 823 is comprised of a combination of the sensoroutputs 821 and 822 muxed at appropriate intervals. FIG. 8B illustratesthat the sensor outputs 821 and 822 are substantially identical andoffset by approximately 180 degrees. The corresponding page flip signal824 selects the interval of each output. The waveforms illustrated arenot drawn to be precise, they can vary depending on the size of thepolarized filter and the distance between sensors. The page flip signal849, when summed with the FB signal 844 at the motor amp input 852,results in the motor drive signal 820 which is also coupled to solenoid801 in the polarity A-B and to solenoid 802 in the inverted polarityD-C, as shown at motor amp output 850 in FIG. 8C. The differentialamplifier 848 takes as its input the summing junction 846 voltage andreference voltage 845. Resistors 847 are provided at the motor ampinputs for the page flip signal, reference, feedback signal, and motoramp output feedback.

Pulse Width Modulation (“PWM”) Embodiment:

The connection from the motor drive amp to the solenoids, in the closedloop embodiment, can be replaced by the PWM modulation embodiment asdescribed below, with reference to FIG. 9. The PWMs 901 and 903 arededicated to driving the solenoids 902 and 905 only while the motor ampdrives only the voice coil actuator. The page flip signal 908 is coupledto the multiplexer and to the motor drive amp and is also fed to the PWMlookup table 907, which lookup table responds to the page flipfrequency. Based on the input frequency, the lookup table selects apulse width, or duty cycle, which is represented as a voltage levelcontrol signal 906 transmitted to the PWMs 901 and 903. In this sense, aPWM is a programmable device having a voltage level control input. Itsoutput is typically a square wave having a particular pulse width. Atypical analog PWM outputs a fixed frequency signal and increases itsduty cycle based on a level of input voltage, typically a DC inputvoltage, in this embodiment provided by lookup table 907. The outputvoltage level 906 of the lookup table 907, in turn, is controlled by afrequency of the input page flip signal 908. The fixed output frequencyof the PWMs 901 and 903 can be arbitrarily selected, for example, in theKHz range so that average the magnetic flux provided by solenoids 902and 905 can be modulated via control of the PWM duty cycle. The PWMoutput frequency should be much higher than the oscillation frequency ofthe polarizer arm. An air core solenoid embodiment provides scalablemagnetic cushioning for the polarizer arm.

The duty cycle of the PWM is determined by the page flip 908 rate inputto the lookup table 907. For example, a 60 Hz detected input mightcorrespond to a 10% duty cycle output by the PWMs as controlled by acorresponding output voltage level provided to the PWMs by the lookuptable. The voltage level received by the PWMs is also recognized ascorresponding to a 10% duty cycle. As a further example, 85 Hz couldcorrespond to a 22% duty cycle and a 120 Hz rate could correspond to a50% duty cycle. These are selectable for best performance of anyparticular system. Typically, a higher page flip frequency will resultin a higher duty cycle output by the PWM. This will, in turn, result inhigher magnetic field strength provided by the solenoids that will berequired in order to better cushion the polarizer arm which is beingdriven at higher speeds due to the higher frequency of the page flipsignals. The outputs to the solenoids by each PWM will have an identicalduty cycle and phase relationship. The magnetic flux at the top andbottom of the solenoids correspond to the north and south pole fields ofa permanent magnet. The output of the lookup table can be selectivelybased on a natural measured resonant frequency of the system and isstored in a memory of the lookup table. Changes in system design (weightof filters, length of filter arm, strength of voice actuator coil, etc)will affect its resonant frequency and the corresponding lookup tablevalues can be adjusted accordingly.

ADVANTAGES OF THE INVENTION

Our invention can operate on projection systems with one, two, or threedigital mirror devices (“DMD”), wherein more devices are used to improvecolor contrast and resolution. Our invention allows for amagnetomechanically oscillating polarizer to be placed outside theprojector. In this way, any existing projector can be modified toprovide a 3D effect.

Our invention includes an embodiment that is capable of substantialextinction between the two polarization states. Since our invention doesnot require modification to the projector filter wheel (which is aprecision balanced component spinning at thousands of RPMs and higher),it becomes significantly easier to implement at lower cost.

Alternative Embodiments

It will be appreciated that, although specific embodiments of theinvention have been described herein for purposes of illustration,various modifications may be made without departing from the spirit andscope of the invention. The entire assembly, as illustrated in FIG. 3C,for example, may be enclosed in an airtight vacuum transparent glasscontainer.

An incoming 2D signal can be switched directly to the DLP system andbypass the 3D imaging circuits by implementing a simple switch at 301.Optionally, the 3D imaging path can be outfitted with detection circuitsthat automatically detect incoming multiple image streams andautomatically send those signals to the 3D generating technology of thepresent invention. Such an automatic activation option could also bemanually disabled. These alternative embodiments are considered to besimple adjustments to the present invention and do not fall outside thescope of the present claims. Accordingly, the scope of protection ofthis invention is limited only by the following claims and theirequivalents.

We claim:
 1. A method comprising the steps of: receiving image datacomprising a plurality of image streams, each of the image streamscomprising a plurality of frames; projecting the image data toward aviewable screen, wherein the projecting step comprises alternatingbetween the image streams; and disposing a magnetomechanicallyoscillating filter comprising first and second type filters between aprojector of the image data and the viewable screen for filtering framesprojected from a first one of the image streams through the first typefilter and filtering frames projected from a second one of the imagestreams through the second type filter; wherein said magnetomechanicallyoscillating filter comprises an oscillating arm and at least twoparallel permanent magnetic plates, the parallel permanent magneticplates comprising a magnetic flux strength resonant at a desiredoperating frequency for the image streams; and wherein the oscillatingarm is magnetically cushioned as it approaches an endpoint of anoscillation path.
 2. The method of claim 1, further comprising the stepsof: filtering from one eye of a viewer the frames from the first one ofthe image streams through another second type filter; and filtering fromthe other eye of the viewer the frames from the second one of the imagestreams through another first type filter.
 3. The method of claim 2,wherein all the filtering steps in combination comprises alternatelyblocking the projected image data from only one eye of the viewer. 4.The method of claim 2, wherein the steps of filtering frames from thefirst one of the image streams and filtering frames from the second oneof the image streams comprise activating a filter mechanism synchronizedwith page flip signals that alternately situates the first type filterand the second type filter in the path of the projected image data. 5.The method of claim 1, wherein the image data contains page flip signalscorresponding to one of the image streams and the step of filteringframes from a first one of the image streams comprises detecting thepage flip signals in the image data that correspond to the first one ofthe image streams.
 6. The method of claim 1, wherein the receiving stepcomprises receiving a series of frames each alternately from each of theplurality of image streams.
 7. The method of claim 1, wherein the stepof filtering frames from the first one of the image streams comprisespolarizing the frames at a first polarization angle, and the step offiltering frames from the second one of the image streams comprisespolarizing the frames at a second polarization angle.
 8. The method ofclaim 7, wherein the first type filter comprises a polarized filter atthe first polarization angle, and the second type filter comprises apolarized filter at the second polarization angle.
 9. The method ofclaim 1, further comprising: detecting a position of the first typefilter; and responsive to the detecting, controlling an oscillation ofthe magnetomechanical oscillating filter.
 10. A television systemcomprising: an input for receiving image data comprising a plurality ofimage streams, each of the image streams comprising a plurality offrames; a light projector for projecting the image data toward aviewable screen, wherein the projecting step comprises alternatingbetween the image streams; and a magnetomechanical oscillating filterfor polarizing the projected image streams each at a differentpolarizing angle, the magnetomechancial oscillating filter comprisingfirst and second type filters disposed between the project and theviewable screen for filtering frames projected from a first one of theimage streams through the first type filter and filtering framesprojected from a second one of the image streams through the second typefilter; wherein said magnetomechanically oscillating filter furthercomprises an oscillating arm and at least two parallel permanentmagnetic plates, the parallel permanent magnetic plates comprising amagnetic flux strength resonant at a desired operating frequency for theimage streams; and wherein the oscillating arm is magnetically cushionedas it approaches an endpoint of an oscillation path.
 11. The televisionsystem of claim 10 wherein the magnetomechanical oscillating filterfurther comprises first and second type filters disposed between theprojector and the viewable screen for filtering frames projected from afirst one of the image streams through the first type filter andfiltering frames projected from a second one of the image streamsthrough the second type filter.
 12. The television system of claim 11,wherein the first type filter is a polarization filter polarized at afirst polarization angle and the second type filter is a polarizationfilter polarized at a second polarization angle.
 13. The televisionsystem of claim 12, wherein the second polarization angle is orthogonalto the first polarization angle.
 14. The television system of claim 10,further comprising: passive eyewear having first and second type filterlenses for filtering from one eye of a viewer the frames from the firstone of the image streams through the second type filter lens and forfiltering from the other eye of the viewer the frames from the secondone of the image streams through the first type filter lens.
 15. Thetelevision system of claim 14, further comprising a field convertercoupled to the input for detecting page flip signals in the image dataand for outputting at least the page flip signals separate from theimage data.
 16. The television system of claim 10, further comprising:page flip signals contained in the image data corresponding to one ofthe image streams and a circuit for detecting the page flip signals forcontrolling the magnetomechanical oscillating filter to align the firsttype filter with the projector when the page flip signal indicates thatimage data corresponds to the first one of the image streams.